Various techniques are conventionally employed in oil and gas well field operations to enhance hydrocarbon recovery. One such technique is hydraulic fracturing of a hydrocarbon-bearing formation to improve hydrocarbon flow from the formation to a producing oil or gas well. In an hydraulic fracturing process or treatment, a fluid, such as a sand-water slurry, is injected into the borehole through a tubing string to the depth interval of interest. The fluid is injected at a rate and pressure sufficient to cause the formation within the selected depth interval to fracture. A propant may then be introduced into the fractured zone to keep the fracture open, thereby enhancing the productivity of the well.
The hydraulic fracturing treatment of oil or gas wells is a time consuming and expensive process, and repeated treatments are sometimes required. Following treatment, substantial additional investments of time and money may well be made in attempting to recover hydrocarbons from the fractured zones. It is important, therefore, that reliable information be available to the well operator regarding the effectiveness of the fracturing treatment. Ideally, this information should be available in situ in real time, i.e., as the fracture event is actually happening in the field.
Prior art techniques for evaluating fracture treatments have included the use of seismic hydrophone arrays, ultrasonic televiewers in the fracture interval, flow meters in the fracture interval, and gamma ray logs after seeding the propant with radioactive tracers. Temperature logs or surveys produced after completion of the treatment, such as those described in U.S. Pat. Nos. 3,480,079, 3,795,142 and 4,109,717, have also been employed. None of these techniques, however, meet the aforementioned need for in situ real time knowledge of fracture growth and extent.
It is an object of the invention, therefore, to provide a method for effectively and reliably monitoring the in situ growth of an hydraulic fracture during the fracturing process.
A further object is to perform the aforementioned monitoring in a way to provide real-time well site information of the fracture growth.
Additionally, an object is to provide a method for the improved evaluation of the production capacity of a fractured zone by providing information of the physical and hydrological properties of the fracture.
Still another object is to monitor the temperature changes in a well over an extended period of time, which could be the lifetime of the well, to facilitate evaluation of the production history of the well.
Still a further object of the invention is to monitor the temperature-vs-depth characteristics of a borehole over time in general, apart from the hydraulic fracture treatment of well bores.